Do you have a link for Cassandra's documentation on map collection type?
Sorry but I can't seem to find a way to bring map data type in.
I don't see any signs of cast options in documentation either and you can only query whole collection entirely (not individual values).
p.s. Forgot to add - skipping is by design. Tableau won't bring in fields it does not know how to handle.
I'd suggest adding an UDF (that can convert map to json string) to your cluster and use that in Custom SQL.